The present invention relates to a method and system for estimating the neutral point of the steering wheel of a vehicle while the vehicle is traveling.
In a vehicle provided with electronic control equipment which is controlled in accordance with a steering condition, the steering wheel, i.e., the steering shaft, is generally equipped with a wheel angle sensor. This wheel angle sensor functions to detect a wheel angle when the steering wheel is steered, for allowing a front-wheel steering angle to be determined from the detected wheel angle.
The steering angle of front wheels provides an important control factor for implementing, for example, rear-wheel steering control or the trace control and the like of a traction control system; therefore, the wheel angle sensor must be capable of detecting the steering angle of front wheels with high accuracy.
The wheel angle sensor is designed to detect a wheel angle when the steering wheel is steered to the right or left from its neutral point, i.e., the front-wheel steering angle. For this reason, a vehicle must be traveling straight ahead whenever the neutral point of the steering wheel is detected through the wheel angle sensor.
If, however, there is an error in the installation of a steering column or a wheel angle sensor, or if a play or the like develops in a steering system extending from the steering wheel to the front wheels as the vehicle is subjected to external forces, then the neutral point of the steering wheel fails to fully coincide with the straight-ahead travel of the vehicle.
Under such circumstances, the front-wheel steering angle detected through the wheel angle sensor becomes inaccurate, making it impossible to carry out optimum rear-wheel steering control or trace control as mentioned above.
As a prior example focusing on such a problem, Japanese Utility Model Applications Laid-Open H2-85675 U.S. Pat. No. 5,018,594, DE3942494, GB2228460) discloses a technology for correcting the neutral point of a wheel angle sensor output on the basis of the output of the wheel angle sensor obtained when the pressure difference between the right and left pressure chambers of a power steering unit is zero.
In an actual vehicle, however, the condition where the zero pressure difference between the right and left pressure chambers of the power steering unit takes place not only at a particular wheel angle but over a certain range of wheel angles. Therefore, the aforesaid method cannot ensure accurate correction of the aforesaid neutral point.
Theoretically, the problem with the above-mentioned prior example can be solved by using a device for accurately detecting whether front wheels are in the straight-ahead travel condition instead of detecting whether the aforementioned pressure difference is zero, however, no such convenient detecting device has been put in practical use.